1. Field of the Invention
The present invention relates to an ink tank having an ink accommodation space therein and capable of supplying ink to the outside of the accommodation space while keeping a pressure in the accommodation space at a predetermined negative pressure at all times.
2. Description of the Related Art
An example of a conventional ink tank of this kind has a negative pressure generation means in the form of a valve structure constructed of a film disk and a spring, as described in Japanese Patent Application Laid-open No. 2003-34041. This ink tank has an ink accommodation portion and an ink supply path formed between the ink accommodation portion and an ink supply port for supplying ink to a print head. In this ink supply path the negative pressure generation means is provided in the form of a valve structure. The valve structure of the negative pressure generation means has its film disk pressed against a valve seat by a biasing force of a spring to normally close the ink supply path. When, as a result of ink ejection from the print head, the negative pressure of ink in the supply path from the supply port to the film disk (a supply path on the supply port side) exceeds a predetermined level, the film disk parts from the valve seat against the biasing force of the spring to open the ink supply path. With the film disk open, ink is supplied from the ink accommodation portion to the supply port through the ink supply path. As a result, the negative pressure in the supply path on the supply port side returns to less than the predetermined level (i.e., the pressure increases) and the film disk is again pressed against the valve seat by the force of the spring to close the ink supply path.
In the conventional ink tank as described above, the negative pressure in the supply path on the supply port side is kept at less than a predetermined level by the open-close action of the film disk to maintain the pressure in the print head communicating with the ink supply path at a negative pressure. As a result, an appropriate meniscus is formed in each of the nozzles of the print head.
The ink tank disclosed in Japanese Patent Application Laid-open No. 2003-34041, however, has the following drawbacks. The use of a spring to bias the film disk increases the number of parts of the ink tank. In terms of assembly, an additional part, spring, may lower the manufacturing efficiency of the ink tank. If the spring is mounted at a deviated position, the biasing force applied to the film disk will vary. This necessitates an additional structure for restricting the spring mounting position. That is, a small component of spring must be assembled precisely by restricting its mounting position, which degrades an assembly efficiency of the ink tank. Furthermore, since the spring is in contact with ink, a material of the spring needs to be chosen which does not degrade the spring function upon contact with ink nor adversely affect the property of ink. An appropriate selection of the spring material is therefore difficult to make and, depending on circumstances, an ink composition may have to be changed.